In general, a small or midsized vehicle has a coupled torsion beam axle (CTBA) suspension system which has been applied to a rear wheel suspension system due to its low production cost, light weight, and a simple component configuration. However, such a CTBA suspension system does not necessarily have superior ride comfort and handling performance compared to an independent suspension system.
FIG. 1 illustrates a perspective view a coupled torsion beam axle suspension system according to a related art.
Referring to FIG. 1, the CTBA suspension system of the related art comprises a torsion beam 1 in a vehicle width direction, and trailing arms 5 respectively secured to both ends of the torsion beam 1 for mounting carriers 3 thereto to which a wheel and a tire are mounted.
The CTBA suspension system of the related art further comprises a spring seat 9 for mounting a spring 7 thereon and a shock absorber pin 13 for connecting a shock absorber 11 thereto at an inner side of a rear side of the trailing arm 5. A vehicle body fastener 15 connects a vehicle body to a front end of the trailing arm 5.
The vehicle body fastener 15 has a trailing arm bush 21 formed on a front end of the trailing arm 5 and fastens a mounting bracket 23 of the vehicle body to the trailing arm bush 21 with a bolt 25.
In the CTBA suspension system of the related art, a torsional deformation characteristic of the torsion beam 1 positioned at a center changes behavior of the wheel, and a position of the trailing arm 5 and a configuration of the vehicle body fastener 15 also change the behavioral characteristic of the wheel.
That is, at the time of cornering of a vehicle, a driver has to maintain an understeer tendency for stable driving. In this case, an outer turning wheel of rear wheels needs to toe-in and an inner turning wheel of the rear wheels needs to toe-out.
However, the CTBA suspension system according to the related art contains the following behavioral problems.
FIG. 2 illustrates a top plan view showing a behavioral characteristic of a CBTA suspension system in which a lateral force is applied thereto.
Referring to FIG. 2, the CTBA suspension system of the related art has zero degree of freedom in view of a mechanism when a lateral force F1 is applied. However, the entire CTBA suspension system may rotate as the trailing arm bush 21 deforms to create a toe angle at an outer turning wheel W1 of the rear wheels. The outer turning wheel W1 has a tendency to toe-out if the lateral force F1 is applied thereto, and an inner turning wheel W2 of the rear wheels, which is rebounded at the time the lateral force F1 is applied thereto, has a tendency to toe-in or maintain a preset toe angle if the lateral force F1 is applied thereto. Thus, the vehicle may oversteer, decreasing cornering stability.
That is, a mechanical instantaneous center of rotation (SP; i.e., a crossing point of extension lines extended in fastened directions of both side trailing arm bushes 21 fastened to the vehicle body) of the CTBA suspension system on the vehicle is positioned in front of both side wheel centers WC, and thus, the outer turning wheel W1 of the rear wheels shows the toe-out tendency on the lateral force F1 and the inner turning wheel W2 shows the toe-in tendency on the lateral force F1.
Recently, in order to solve the corner stability problem of the CTBA suspension system, a vehicle body fastening structure of the vehicle body and the trailing arm 5 has been improved to position the instantaneous center of rotation (SP) behind both side wheel centers WC.
FIG. 3 illustrates a top plan view of a CTBA suspension system according to another exemplary related art.
Referring to FIG. 3, in order to position an instantaneous center of rotation (SP) on a vehicle behind both side wheel centers WC, the CTBA suspension system has a bush link 31 applied between a vehicle body and a trailing arm bush 21 as a vehicle body fastener 15 for fastening to the vehicle body.
That is, the bush link 31 has a rear end fastened to the trailing arm bush 21 parallel to the vehicle width and a front end with a lower mounting bush 33 provided thereto to have rotational degrees of freedom with respect to the vehicle body fastened to one side of a lower portion of the vehicle body.
In this case, the lower mounting bush 33 is formed on the bush link 31 such that the lower mounting bush 33 is fastened to the vehicle body in front of an outer side of the trailing arm bush 21 in the vehicle width direction, and is fastened to the vehicle body in a vehicle height direction.
The instantaneous center of rotation (SP) of the CTBA suspension system on the vehicle body is formed at a crossing point of extension lines which connect centers S1 of the lower mounting bushes 33 and centers S2 of the trailing arm bushes 21, respectively, to position the instantaneous center of rotation (SP) behind the both side wheel centers WC.
Since the CTBA suspension system according to the related art has the instantaneous center of rotation (SP) positioned behind both side wheel centers WC, it has the following behavioral characteristics on the lateral force F1 and a front/rear force F2.
FIGS. 4A to 4C illustrate top plan views of behavioral characteristics on a lateral force F1 and a front/rear force F2 applied to the CTBA suspension system, respectively according to the related art.
Referring to FIG. 4A, in the CTBA suspension system of the related art, if the lateral force F1 is applied to both rear wheels like at the time of the corner driving of the vehicle, the outer turning wheel W1 of the rear wheels is induced to toe-in and the inner turning wheel W2 is induced to maintain a preset toe angle or toe-out, thus making the vehicle understeer and enabling secure cornering stability.
When the lateral force F1 and the front/rear force F2 are both applied to the rear wheels, the CTBA suspension system is induced to turn with reference to the instantaneous center of rotation (SP).
That is, as shown in FIG. 4B, since the CTBA suspension system has the rear wheels rotating in symmetry to cancel an entire rotation effect of the CTBA suspension system in a double impact situation in which the front/rear force F2 is applied to the both side rear wheels when a driver applies a brake or the vehicle passes a speed bump, the cornering stability is secured.
However, as shown in FIG. 4C, in a single impact situation in which the front/rear force F2 is applied only to one of the two rear wheels when driving on a rough road to induce the wheel to toe-out, the entire CTBA suspension system behavioral characteristic becomes unstable, and the CTBA suspension system still has a problem of poor driving stability.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.